Andrew J. Sansom

Intraventricular injection of an N-methyl-D-aspartate antagonist disrupts vestibular compensation

Guinea pigs which had compensated for a unilateral labyrinthectomy exhibited a loss of ocular motor and postural compensation when a 40 or 20 mM concentration of the NMDA antagonist CPP, was injected through a cannula implanted in the IVth ventricle close to the vestibular nuclei. Similar injections of artificial cerebrospinal fluid did not induce loss of compensation. These results suggest that NMDA receptors may contribute to vestibular compensation in the guinea pig.

Evidence for reduced nitric oxide synthase (NOS) activity in the ipsilateral medial vestibular nucleus and bilateral prepositus hypoglossi following unilateral vestibular deafferentation in the guinea pig

The aim of the present study was to examine, using a radioenzymatic assay technique, nitric oxide synthase (NOS) activity in the bilateral medial vestibular nuclei (MVN) and prepositus hypoglossi (PH), during the development of vestibular compensation for unilateral vestibular deafferentation (UVD) in the guinea pig. In the MVN ipsilateral to the UVD, and bilaterally in PH, NOS activity decreased following UVD compared to sham controls and did not recover significantly up to 50 h later, when a substantial degree of behavioural vestibular compensation had occurred. These results suggest that UVD causes a decrease in NOS activity in the ipsilateral MVN and the bilateral PH, and that a consequent decrease in NO may be responsible for some of the ocular motor and postural symptoms of UVD.

Pretreatment with MK-801 reduces spontaneous nystagmus following unilateral labyrinthectomy

Unilateral labyrinthectomy results in a syndrome of ocular motor and postural symptoms which abate over time in a process of behavioural recovery known as vestibular compensation. We have previously suggested that an increased Ca2+ influx in ipsilateral vestibular nucleus (VN) neurons at the time of the unilateral labyrinthectomy may exacerbate the depression of VN resting activity caused by the loss of excitatory input from the VIIIth nerve. In order to further test this hypothesis, we administered (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801; 1.0 or 2.5 mg/kg i.p.), which blocks Ca2+ influx via NMDA receptor-mediated ion channels, to guinea pigs 0.5 h before unilateral labyrinthectomy and examined the effects on three symptoms of unilateral labyrinthectomy: spontaneous ocular nystagmus, yaw head tilt and roll head tilt. Pretreatment with MK-801 significantly altered the time course of the vestibular compensation of spontaneous nystagmus and yaw head tilt but had no significant effect on roll head tilt; in particular, 2.5 mg/kg MK-801 depressed spontaneous nystagmus frequency at 10 and 20 h post-labyrinthectomy relative to saline controls (P less than 0.05, post-hoc Scheffé F-test). The reduction in spontaneous nystagmus frequency was not simply a result of extended anesthesia, since other control animals, which received additional injections of the general anesthetic in order to achieve equivalent sleep times to the MK-801 group, did not show reduced spontaneous nystagmus frequency.(ABSTRACT TRUNCATED AT 250 WORDS)

An antisense oligonucleotide to brain-derived neurotrophic factor delays postural compensation following unilateral labyrinthectomy in guinea pig.

An antisense oligonucleotide to brain-derived neurotrophic factor (BDNF) was delivered by osmotic mini-pump at a 1 mM concentration via a cannula into the ipsilateral vestibular nucleus complex from 15 to 56h following unilateral labyrinthectomy in guinea pigs. Compared with the control groups, vestibular compensation of roll head tilt was significantly delayed (p < 0.05), while compensation of spontaneous nystagmus and yaw head tilt was unaffected. These results suggest that neurotrophins such as BDNF may be involved in specific aspects of the vestibular compensation process.

In vitro phosphorylation of medial vestibular nucleus and prepositus hypoglossi proteins during behavioural recovery from unilateral vestibular deafferentation in the guinea pig.

Unilateral removal of vestibular nerve input to the vestibular nuclei (e.g. by unilateral labyrinthectomy, UL) results in severe ocular motor and postural disorders which disappear over time (vestibular compensation). We investigated whether recovery of ocular motor function is temporally correlated with changes in protein phosphorylation in the medial vestibular nucleus (MVN) and prepositus hypoglossi (PH; MVN/PH) in vitro. Bilateral MVN/PH were dissected from 48 guinea pigs following decapitation at 10 h, 53 h or 2 weeks post-UL, or -sham operation and frozen. Tissue extracts were incubated with [gamma-32P]ATP +/- Ca2+ plus phorbol 12,13-dibutyrate and phosphatidylserine. UL resulted in a significant bilateral increase in the 32P-incorporation into a 65-85 kDa band (probably the myristoylated alanine-rich C kinase substrate, MARCKS) in compensated animals (53 h post-UL) under conditions which favoured the activation of protein kinase C. Under identical conditions, the labelling of a 42-49 kDa protein (P46) was increased significantly in the bilateral MVN/PH between either 10 h or 53 h and 2 weeks post-UL; there were no significant changes over time in sham controls. These results show that later stages of vestibular compensation are accompanied by changes in the phosphorylation of several likely protein kinase C substrates in the MVN/PH in vitro.

The effects of steroids on vestibular compensation and vestibular nucleus neuronal activity in the guinea pig.

Recent studies have suggested that steroids such as dexamethasone and methylprednisolone might be useful in the treatment of vestibular disorders, irrespective of whether inflammatory processes are involved. The aim of this study was to investigate the effects of systemic administration of dexamethasone on vestibular compensation of spontaneous nystagmus (SN) in guinea pig, and the effects of dexamethasone and methylprednisolone on extracellularly recorded spontaneous activity of medial vestibular nucleus (MVN) neurons in brainstem slices in vitro. In the behavioral study, none of the 3 doses of dexamethasone (5, 10, or 40 mg/kg i.p., delivered at 0, 12, 24, and 36 h following a unilateral surgical labyrinthectomy (UL)) resulted in a significant change in the frequency or compensation of SN, relative to the vehicle control group. In the in vitro study, only a minority of MVN neurons showed any response to 1 microM dexamethasone (1 out of 9 neurons), or 10 nM (3 out of 13), or 0.1 microM methylprednisolone (3 out of 7). These results suggest, contrary to previous evidence, that dexamethasone may not accelerate compensation of SN following surgical UL and that dexamethasone and methylprednisolone may have a direct action only on a minority of MVN neurons.

The opioid receptor antagonist, naloxone, enhances ocular motor compensation in guinea pig following peripheral vestibular deafferentation.

The opioid receptor antagonist, naloxone, has been demonstrated to enhance recovery from spinal cord injury and fluid percussion brain injury. The present study investigated, for the first time, the effects of naloxone on behavioral recovery following unilateral peripheral vestibular deafferentation (unilateral labyrinthectomy, UL) in guinea pig. An ip injection of 5 mg/kg naloxone 30 min pre-UL and 5 h post-UL was found to significantly reduce the frequency of spontaneous nystagmus relative to the vehicle control group (P < 0.005). However, a lower dose (2.5 mg/kg) had no effect. At either dose, the effects on the postural symptoms, yaw head tilt and roll head tilt, were small by comparison and in most cases nonsignificant. These results suggest that naloxone can reduce the ocular motor effects of UL in a dose-dependent fashion.

Injections of calmidazolium chloride into the ipsilateral medial vestibular nucleus or fourth ventricle reduce spontaneous ocular nystagmus following unilateral labyrinthectomy in guinea pigs.

The effects of three injections (0.5–4.5 h postoperation) of 1-[bis-(p-chlorophenyl)methyl]-3-[2,4-dichloro-beta-(2, 4-dichlorobenzyloxy)phenethyl]-imidazolium chloride (calmidazolium chloride, R24571), into the ipsilateral medial vestibular nucleus or fourth ventricle, on vestibular compensation for unilateral labyrinthectomy was studied in guinea pigs. R24571, a calmodulin antagonist and inhibitor of several Ca2+-dependent enzymes, caused a significant reduction in the average frequency of spontaneous ocular nystagmus (spontaneous nystagmus) during the first 53 h following unilateral labyrinthectomy (n= 5), compared with vehicle-injected animals (n=5). Although a statistical analysis was not performed on the yaw head tilt and roll head tilt data because of the large variability between animals over the 53-h period of compensation, most R24571 treated animals had less yaw head tilt (4/4 animals) and roll head tilt (4/5 animals) at 9–11 h post-labyrinthectomy than the average values for the vehicle groups at that time. The decrease in the frequency of spontaneous nystagmus following R24571 treatment was not associated with general ataxia or sedation. These results are consistent with recent biochemical studies in suggesting that intracellular pathways associated with Ca2+ may be involved in the neuronal mechanisms of vestibular compensation following unilateral labyrinthectomy.

Fentazin anesthesia for labyrinthine surgery in guinea pigs.

We describe the use of fentazin anesthesia (0.4 mg/ml fentanyl citrate, 58.3 mg/mL xylazine HCl, and 3.2 mg/mL azaperone, Parnell, New Zealand) for labyrinthine surgery in guinea pigs. Fentazin provides stable surgical anesthesia for approximately 1 hour, rapid recovery from anesthesia following the surgery, and has significant analgesic and anxiolytic properties. Its advantages in comparison to other anesthetics (for example, ketamine HCl) are discussed.

Simple device for quantifying drug effects on the righting reflex.

A simple, inexpensive device is described that allows quantification of the effects of drugs on the righting reflex. This device consists of a modified set of kitchen scales connected to a digital timer. Two moveable Hall effect switches are positioned around the pointer, which registers the weight of the animal on the scales; when the animal is placed on the scales in the supine position, the initiation of a righting reflex causes the pointer to cross one of the switches, stopping the digital timer and providing a measure of righting reflex latency (RRL). We describe an efficient protocol for using this device that provides quantification of drug effects on the RRL, which can then be subjected to analysis using parametric statistics such as analysis of variance.